1. Field of the Invention
The present invention relates to a color image forming method capable of recording a sharp color image with a high density and an apparatus therefor, or a color ink jet recording method, and more particularly to a color ink jet recording method utilizing color inks for example of yellow (Y), magenta (M) and cyan (C) or green (G), red (R) and blue (B) and black (Bk) ink.
The present invention is applicable to any equipment utilizing a recording medium such as paper, fabric, non-woven cloth or a overhead projector sheet, and the examples of such equipment include office equipment such as a printer, a copying machine or a facsimile apparatus, and industrial equipment utilized for mass production.
2. Related Background Art
The ink jet recording has been utilized in printers, copying machines, facsimile apparatus etc. because of the advantages of low noise, low running cost, ease of compactization and ease of color image formation.
In general, the ink jet recording achieves formation of a color image by the use of three color inks of cyan, magenta and yellow, or of four color inks additionally including black ink.
More specifically, in the conventional color ink jet recording apparatus, there are known a method which does not utilize black ink but forms black color by mixing yellow, magenta and cyan color inks or green, red and blue color inks in an area to be recorded black, and a method which applies black ink, prepared in advance, to the black recording area.
These conventional color recording methods are merely compared as to the preferability of the black color obtained by three color inks and that obtained by black ink, but the black ink and the three color inks are assumed to be of comparable properties.
For obtaining a color image of high development without ink blotting, the conventional ink jet recording methods have required an exclusive sheet provided with an ink absorbing layer. Recent improvements in ink have realized printing on so-called "plain paper" which is commonly utilized in the printers and copying machines. However, the print quality (or dignity) on such plain paper is still in an insufficient level. A major factor for such insufficiency lies in the balance between the mutual blotting of color inks and the black recording quality (particularly black character recording quality).
In forming a color image on plain paper by ink jet recording, there are generally employed rapid-drying inks having a high penetration speed into the plain paper. There is thus obtained a high-quality image without mutual blotting among the color inks, but such image is generally deficient in the overall density, and, around the recorded image area of each color, there is often observed so-called feathering phenomenon, or slight ink blotting along the paper fibers.
Such feathering is relatively non-apparent in the color image area, but becomes conspicuous in the black image area, leading to deterioration of the recording quality. Particularly when the black image is a black character, the character appears unsharp and unclear, regarded as of a poor quality.
Therefore, for obtaining a black recording of high quality with reduced feathering, there has to be used black ink with a relatively low penetration speed in the plain paper. In such case, however, at the boundary between a black recorded area and a recorded area of another color, there is generated blotting of the ink of said another color, thereby significantly deteriorating the print quality.
Also the conventional ink jet recording methods have been associated with the following drawback.
The ink employed in the conventional ink jet recording is principally composed of water, to which is added a water-soluble high-boiling solvent such as glycol, for the purposes of prevention of ink drying and of nozzle blocking. Recording on plain paper with such ink may give rise to an insufficient image density, due to the penetration of ink into the interior of the recording paper, or an uneven image density presumably due to an uneven distribution of the filler or the sizing material on the paper surface. Particularly in case of forming a color image, since plural color inks are superposed in succession prior to the image fixation, there are observed color blotting and uneven color mixing (hereinafter called bleeding) at the boundary of image areas of different colors, so that a satisfactory image cannot be obtained.
For resolving such drawbacks, the Japanese Patent Laid-open Application No. 55-65269 discloses the use of ink containing a compound enhancing the penetrating ability of the ink, such as a surfactant, and the Japanese Patent Laid-open Application No. 55-66976 discloses the use of ink principally composed of volatile solvent. However, although the former method can suppress the bleeding to a certain extent, the increased penetrating property of ink into the recording sheet causes the ink to penetrate with the colorant into the deep interior of the recording paper, thus resulting in a lowered image density or a deteriorated image sharpness. Also the enhanced wetting property to the paper surface ink spreading, thus leading to a lowered resolution or ink blotting, and the loss of print quality is undesirable particularly in the case of forming a black character. Also the latter method is associated, in addition to the above-mentioned drawbacks, with the nozzle blocking due to the solvent evaporation at the nozzles of the recording head.
As explained in the foregoing, the prevention of bleeding of the inks of black and other colors and the establishment of black print quality, or the prevention of blotting of the inks of black and other colors and the reduction of feathering particularly in the black ink are mutually incompatible requirements in improving the color recording quality. For meeting these requirements, there have been proposed following recording methods.
The Japanese Patent Laid-open Application No. 3-146355 proposes a method of not recording an area along the boundary between a black image area and a color image area. This method, however, is associated with a drawback that the data to be recorded are subjected to a variation.
Also the Japanese Patent Laid-open Application No. 4-158049 proposes a method of utilizing recording heads of plural colors for color recording and a recording head for character recording, which are switched according to the image to be recorded. This method, however, requiring the character recording head in addition to the conventional plural heads for color recording, is inevitably associated with an increase in the cost and in the dimension of the apparatus.
Also the Japanese Patent Laid-open Application No. 4-147866 proposes a recording method in which the fixing time for color inks is made shorter than that for black ink and shorter than a predetermined time. In this method, however, unfixed black ink flows into the area of a previously fixed color ink, thereby making worse the bleeding between the black color and other colors, so that a satisfactory image cannot be obtained.